Here's the promised SVP meeting abstract, _emphasis_ (italics) in the original:
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Vera Weisbecker, Anjali Goswami, Marcelo Sánchez-Villagra & Stephen Wroe:
Postcranial sequence heterochrony and the marsupial-placental dichotomy, SVP
meeting abstracts 2007, 164A
It has long been recognized that there is a deep dichotomy between marsupials
and placentals in terms of taxonomic and ecological diversity. Marsupial
species known today comprise a small percentage of the number of placental
species, and have no extremely specialized (e.g. aerial or fully aquatic)
representatives. The comparative lack of marsupial diversity is particularly
eident in the marsupial forelimb, which is much less diverse compared to that
of placentals. Marsupials are born in an extremely altricial state, and in most
species, the neonate actively crawls to the pouch using its forelimbs. Current
consensus holds that the crawl to the pouch is related to heterochronic
acceleration of the marsupial forelimb. Howeve, this hypothesis has only
experienced limited formal testing. We analysed postcranial ontogeny in a new
dataset of ossification sequences of 13 placentals, 11 marsupials, and three
outgroup taxa, considering 25 events. Data were obtained from clear-stainin!
g and/or
micro-computed tomography of ontogenetic series, and in some cases from the
literature. The ossification sequence data were analyzed using the
event-pair[-]based Parsimov analysis. To examine their potential phylogenetic
signal, a parsimony analysis of the event-pair data was conducted. This did not
retrieve any of the recognized mammalian clades except for monophyletic
marsupials (except *Petaurus*) nested within placentals. This suggestes that
postcranial ossification sequences carry some limited phylogenetic signal for
marsupials, although [this is] not sufficient to separate them from placentals.
Event distributions within species suggest that a larger amount of events
occurs in younger animals compared to older specimens, a pattern particularly
obvious in marsupials. We expect[ed?] that this is due to fast appearance of
the forelimb long bones, shoulder girdle, cervical and thoracic elements in the
youngest marsupials. Parsimov analysis reports that ossification events!
in the
hindlimb of marsupials are _decelerated_ with respect to forelimb long bone
ossification events. This novel ypothesis challenges the long-held tenet of an
ontogenetic "bottleneck" on marsupial forelimb diversity.
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So, it's not the brain that the placentals develop while the marsupials are
developing their forelimbs. It's the hindlimbs.
Marsupials don't develop the forelimbs earlier than placentals, they develop
the hindlimbs later than placentals.
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